Abstract : Using MPI as communication interface, one or several applications may introduce complex communication behaviors over the network cluster. This effect is increased when nodes of the cluster are multi-processors, and where communications can income or outgo from the same node with a common interval time. Our goal is to understand those behaviors to build a class of predictive models of bandwidth sharing, knowing, on the one hand the flow control mechanisms and, on the other hand, a set of experimental results. This paper present experiences that show how is shared the bandwidth on Gigabit Ethernet, Myrinet 2000 and Infiniband network before to introduce the models for Gigabit Ethernet and Myrinet 2000 networks.